South Jersey Skies: The darkest parts of the Moon

There are places on our Moon where the Sun never shines. These may be important someday.

There are no places on Earth that have a wide view of the sky but still never see the Sun. It is true that the Arctic and Antarctic regions have six months of continuous darkness, but that's always followed by six months of solid daylight.

This is due to the 23.5-degree tilt of the Earth's rotation axis. During half the year the Northern Hemisphere is tilted toward the Sun; for the remainder, it's tilted away. If the Earth weren't tilted, the Sun would always be right on the horizon for Santa Claus and other residents of either pole.

There are worlds that have very little tilt. Venus is one. So is the Moon. Our satellite is tipped by only 1.5 degrees. From a vantage point at the lunar North or South Pole, the Sun is never more than two degrees above or below the horizon.

But that's true only if you have a smooth, level horizon. That's generally not the case for the Moon: it's covered with craters, mountains, and valleys.

In particular, the lunar South Pole sits on the rim of a 12-mile-wide crater called Shackleton (after the explorer of the Earth's South Pole). That crater lip is more than two degrees high as seen from its center. This means if you lived on the floor of Shackleton crater (and you may someday, if you're young enough), the Sun would never get high enough to peek over the edge. This location is always in darkness.

View full sizeNASA / GSFC / Arizona State Univ. / Lunar Reconnaissance OrbiterIllumination map of the Moon's South Pole. The brightness of each point on this graphic indicates how often that point is in sunlight. White pixels are always illuminated by the Sun; black pixels, never.

Recently the Lunar Reconnaissance Orbiter painted a picture to illustrate the situation. It took more than 1,700 images of the South Polar region over a period of about six months (which is also about six lunar days, since the Moon rotates so slowly).

For each point on the lunar surface within two degrees of the South Pole, on each photo, astronomers noted whether that point was in sunlight or darkness. Then they drew a special map of the area, making each pixel a shade of gray depending on what fraction of the time that point on the surface was illuminated.

Most points were gray: sometimes they are sunlit, sometime not. But some points, such as those at the bottom of Shackleton crater, were pure black.

The result can be seen on a recent Astronomy Picture of the Day. Take a look at apod.nasa.gov/apod/ap110423.html. This looks like a rather weird snapshot of the lunar surface, but it's not. It's an “illumination map.” Where a pixel is gray, that location received sunlight during some photos but not all. Where a pixel is pure black, that point never saw the Sun.

These may be important places when we start living and working on the Moon. We might find water ice there. Elsewhere, ice that might otherwise be present would vaporize in direct sunlight and be lost. But ice might survive in the permanently dark places.

We might be able to melt the ice to obtain water and even oxygen. Core samples taken down through any ice we find, like core samples taken in the Antarctic, might help us understand the past history of the Moon.

You may notice some points on the photo that are pure white, or nearly so. For instance, the rim of Shackleton crater is bright, meaning it basks in sunlight nearly all the time (though the Sun is always very low in the sky).

This might be a good place to mount our solar collectors, giving future lunar colonies electrical power on nearly a constant basis.

Keith Johnson is the director of Rowan University's Edelman Planetarium, which is showing the full-dome movie “Two Small Pieces of Glass” at 7 p.m. on Saturday nights through May 7. Teachers who plan to take their classes on field trips to the planetarium should send in their requests soon, as the rest of the school year is filling up fast. Email him at johnsonk@rowan.edu, or call him at 856-256-4389.

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